The formation of chromium-free conversion layers on the metal surfaces mentioned is covered by extensive prior art literature as cited, for example, in WO 94/28193.
It is clear from many of the documents cited therein that the conversion layers formed are colorless and transparent, so that the treated metal surfaces are bright in appearance. However, from many years' experience in the chromating of metal surfaces, the expert in this field is accustomed to obtaining a colored layer as the outcome of the conversion treatment. He is then able to see immediately whether the treatment has been successful. However, in the production of colorless layers, more elaborate surface analysis is required for this purpose, for example determination of the Ti content of the surface by X-ray fluorescence measurement. Accordingly, there is a need in practice for surface treatment processes which not only match conventional chromating layers in regard to corrosion prevention and paint adhesion properties, but, like chromating layers, are also visible to the human eye.
Attempts to solve this problem can be found in the prior art. For example, WO 94/25640 describes a process for producing blue-colored conversion layers on zinc/aluminium alloys. In this process, the metal surfaces are contacted with a treatment solution which has a pH value of 3.5 to 6 and which contains 0.2 to 3.0% by weight molybdenum and 0.1 to 2.0% by weight fluoride. The molybdenum may be used as molybdate, as phosphomolybdic acid, as molybdenum chloride and the like. The fluoride may be used in the form of hydrofluoric acid, simple fluorides and also complex fluoro acids, such as fluorotitanic acid or fluorozirconic acid for example.
A similar treatment process is described in WO 95/14117 although the appearance of the conversion layers is not discussed in detail. According to this document, the treatment solution contains hetero-oxo anions of molybdenum, tungsten or vanadium with one of the heteroions phosphorus, aluminium, silicon, manganese, zirconium, titanium, tin, cerium or nickel. In addition, the treatment solution contains an organic film former which may be selected, for example, from acrylates. The hetero-oxo anions, such as anions of heteropolyacids for example, may be directly formed in the treatment solution by addition of the relevant starting materials, for example molybdate ions and phosphoric acid, to the treatment solution. In addition, the treatment solution is said preferably to contain an etching reagent for aluminium, for example fluoride, tetrafluoroborate or similarly acting etching reagents.
According to the teaching of WO 00126437, the conversion layer is colored by an organic dye (alizarin dye). The conversion layer itself is formed with a treatment solution which contains complex fluorides, for example of titanium and zirconium, besides other inorganic oxides, hydroxides or carbonates or reaction products thereof with the fluoro acids. In addition, a poly-4-hydroxystyrene (polyvinyl phenol) substituted by amino groups may be present as an organic polymer.
According to FR-A-2 461 764, colored layers can be formed on aluminium by chemical oxidation with aromatic nitro compounds. For example, the various position isomers of nitrobenzoic acid may be used for this purpose. The chemical oxidation of aluminium surfaces with persulfate ions is described in DE-A-741 337. However, the goal there is to produce glass-clear and colorless layers on aluminium and its alloys. According to the document in question, such layers are produced by allowing ammonia in the form of a hot aqueous solution additionally containing added alkali metal persulfate (preferably 1%) to act on the carefully cleaned light metal. Cleaning can be carried out, for example, by immersion in concentrated nitric acid. The treatment with the ammoniacal persulfate solution is carried out at temperatures of at least 70° C. over a period of about 15 to 60 minutes. It is pointed out that protective layers are also obtained at temperatures below 70° C., but are less permeable to light and often tinged with yellow. In the context of the disclosure of the cited document, yellow-tinged layers such as these are clearly undesirable because the treated parts are not intended to be painted, but to retain their metallic appearance.